Wooden roof truss

ABSTRACT

A roof truss is provided comprising a bottom chord, a first top chord, and a second top chord forming a triangle with an apex at a distance A above the bottom chord. At least a web member is disposed between the top chords and the bottom chord such that, in a first state of operation, the at least a web member is capable of transmitting at least one of a tension force and a compression force between the top chords and the bottom chord. The roof truss further comprises at least an expandable interface for expandable interfacing at least one of the at least a web member with one of the chords for enabling, in a second state of operation, expansion of the distance A to an extent that uplift of the bottom chord is substantially reduced.

FIELD OF THE INVENTION

This application claims the benefit of U.S. Provisional PatentApplication No. 61/064,305 filed Feb. 27, 2008, the entire contents ofwhich are incorporated herein by reference.

The instant invention relates to the field of wooden roof trusses and inparticular to a wooden roof truss having an expandable interface forsubstantially reducing roof truss uplift.

BACKGROUND

Most present-day roof trusses form a triangular structure comprising abottom chord and two inclined top chords which meet in an apex at apredetermined distance—rise—above the bottom chord. The bottom chord andthe top chords are joined by web members forming triangularsubstructures of the roof truss. Each of the web members transmits oneof a tension force and a compression force between one of the two topchords and the bottom chord depending on load acting on the roof truss.

Air in a well-ventilated attic space contains approximately a sameamount of moisture as outside air. In winter the relative humidity ofthe outside air is relatively high, therefore, the top chords and webmembers absorb moisture until equilibrium is reached with the outsideair. Consequently, the top chords and the web members lengthen.

The bottom chord, however, experiences a different phenomenon. In orderto meet building code requirements of colder climate zones, builderscover the bottom chord with an approximately 300 mm thick layer ofinsulating material. Therefore, the average temperature surrounding thebottom chord is close to the indoor temperature. This causes the airadjacent to the bottom chord to have a much lower relative humidity thanthe air outside the layer of insulating material. As a result, the airadjacent to the bottom chord absorbs moisture from the wood causing thebottom chord to shorten.

As the bottom chord shortens and the top chords lengthen—which is notcompensated by the lengthening of the web members—the apex of the rooftruss is forced upward. Thus, web members connected to the top chordsnear the apex pull the bottom chord upward resulting in a roof trussuplift causing cracks of up to approximately 20 mm width betweenceilings and partitioning walls. It is worth noting, that in case thechords and the web members are made of compression wood or juvenilewood, this effect is significantly increased.

Some builders mask the effects of the roof truss uplift by securing theceiling drywall to the top of the partitioning walls and not to the rooftrusses for a distance of approximately 45 cm from the partitioningwalls. The drywall flexes and stays fastened to the partitioning wallswhile the trusses lift above it. Unfortunately, this method leaves aconsiderable portion of the ceiling drywall without support.Furthermore, mounting of fixtures such as hanging light fixtures to thisportion of the ceiling is difficult if not impossible.

Others fasten decorative moldings to the ceilings along edges where thepartitioning walls and the ceilings meet. As the ceilings move up, themoldings move therewith, thus hiding the gap. Unfortunately, sincecolors change when exposed to sunlight this will expose an undecorativestripe below the molding in winter. Furthermore, it is suggested toalways redecorate in winter when the ceiling is at its highest point.Otherwise a stripe will be exposed below the molding during thefollowing winter.

It would be highly desirable to overcome these drawbacks and tosubstantially reduce roof truss uplift.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In accordance with an aspect of the present invention there is provideda wooden roof truss comprising:

a bottom chord, a first top chord, and a second top chord, the bottomchord, the first top chord, and the second top chord joined togetherforming a triangle with an apex spaced at a distance A from the bottomchord;a web member disposed between the top chords and the bottom chord suchthat, in a first state of operation, the a web member transmits one of atension force and a compression force between the top chords and thebottom chord; and,an expandable interface for expandably interfacing the web member withone of:

-   -   the first top chord;    -   the second top chord;    -   the first and the second top chord; and,    -   the bottom chord,        wherein the expandable interface supports, in a second state of        operation, variation of the distance A between the apex and the        bottom chord.

In accordance with an aspect of the present invention there is furtherprovided a wooden roof truss comprising:

a bottom chord, a first top chord, and a second top chord, the bottomchord, the first top chord, and the second top chord joined togetherforming a triangle with an apex spaced at a distance A from the bottomchord;a web member disposed between at least one of the top chords and thebottom chord such that, in a first state of operation, the web membertransmits one of a tension force and a compression force between the atleast one of the top chords and the bottom chord, wherein the web membercomprises an expandable interface disposed between a first and a secondportion of the web member for supporting relative movement of the firstportion with respect to the second portion substantially along alongitudinal axis of the web member for supporting, in a second state ofoperation, variation of the distance A between the apex and the bottomchord.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments of the invention will now be described inconjunction with the following drawings, in which:

FIGS. 1 a to 1 e are simplified block diagrams illustrating variousexamples of triangular wooden roof trusses according to embodiments ofthe invention;

FIGS. 2 to 4 are simplified block diagrams illustrating triangularwooden roof trusses according to embodiments of the invention;

FIGS. 5 a and 5 b are simplified block diagrams of a first embodiment ofan expandable interface according to the invention;

FIGS. 6 a to 6 k are simplified block diagrams of a second embodiment ofthe expandable interface according to the invention;

FIGS. 7 a to 7 d are simplified block diagrams of a third embodiment ofthe expandable interface according to the invention;

FIGS. 8 a to 8 c are simplified block diagrams of another expandableinterface according to an embodiment of the invention;

FIGS. 9 a to 9 g are simplified block diagrams of a fourth embodiment ofthe expandable interface according to the invention; and,

FIGS. 10 a to 10 g are simplified block diagrams of various limitingmechanisms for use with the expandable interfaces according toembodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following description is presented to enable a person skilled in theart to make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the scope ofthe invention. Thus, the present invention is not intended to be limitedto the embodiments disclosed, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Referring to FIGS. 1 a to 1 e, simplified block diagrams illustratingvarious examples of triangular wooden roof trusses 100A to 100Eaccording to embodiments of the invention are shown. The example woodenroof trusses 100A to 100E according to embodiments of the invention arerelated to standard roof truss shapes which are:

King Post roof truss, shown in FIG. 1 a;Queen Post roof truss, shown in FIG. 1 b;Fink roof truss, shown in FIG. 1 c ;Howe roof truss, shown in FIG. 1 d; and,Fan roof truss, shown in FIG. 1 e.

While the embodiments of the invention will be described based on thestandard roof truss shapes shown in FIGS. 1 a to 1 e for the sake ofsimplicity, it will become evident to those skilled in the art that itis possible to implement the invention in various other roof trussshapes such as, for example, Modified Queen Post, Double Fink, DoubleHowe, Double Fan, and Dual Pitch, as well as non-standard roof trussshapes.

As shown in FIGS. 1 a to 1 e, each of the example roof trusses 100A to100E comprise a bottom chord 10, a first top chord 12, and a second topchord 14. The bottom chord 10, the first top chord 12, and the secondtop chord 14 form a triangle with an apex 16 at a distance A above thebottom chord 10. A different number of web members are disposed betweenthe top chords—12 and 14—and the bottom chord 10 depending on the rooftruss shape, i.e. one web member 20 in the case of the King Post rooftruss; three web members 20.1 to 20.3 in the case of the Queen Post rooftruss; four web members 20.1 to 20.4 in the case of the Fink roof truss;five web members 20.1 to 20.5 in the case of the Howe roof truss; andsix web members 20.1 to 20.6 in the case of the Fan roof truss. The webmembers 20, 20.x are disposed such that, in a first state of operation,they transmit a tension force or a compression force between the topchords—12 and 14—and the bottom chord 10. The first state of operationrefers to a state where the top chords—12 and 14—as well as the bottomchord 10 are exposed to a same level of relative humidity, while asecond state of operation refers to a state where the top chords—12 and14—are exposed to a substantially higher level of relative humidity thanthe bottom chord, i.e. the top chords—12 and 14—are lengthened while thebottom chord 10 is likely shortened. In order to compensate thelengthening of the top chords—12 and 14—at least one expandableinterface 102 is used for expandably interfacing at least one of the webmembers, as shown in FIG. 2 for the simplest case of the King Post rooftruss. As shown in FIG. 2 lengthening of the top chords—12 and 14—causesan upward movement of the apex 16, i.e. the distance A increases fromdistance A(1) to distance A(2). By using the expandable interface 102 inthe wooden roof truss according to an embodiment of the invention,expansion of the distance A is enabled and, therefore, uplift of thebottom chord is substantially reduced, as shown in FIG. 2.

As is evident, there are numerous possibilities of placing theexpandable interface 102 in the wooden roof truss according to theinvention. Regarding the King Post roof truss shown in FIG. 1 a, theexpandable interface 102 optionally is placed at intersection I of theweb member 20 with the bottom chord 10 or, alternatively, atintersection II of the web member 20 with the top chords 12 and 14.Regarding the Queen Post roof truss shown in FIG. 1 b, the expandableinterface 102 optionally is placed at intersection I of the three webmembers 20.1 to 20.3 with the bottom chord 10, at intersections II, III,and IV of the three web members 20.1 to 20.3 with the top chords 12 and14 or, alternatively, only at intersection II of the web member 20.2with the top chords 12 and 14. Depending on the size, the design of theQueen Post roof truss and the lengthening of the top chords, it ispossible to substantially reduce the uplift of the bottom chord 10 byusing only one expandable interface 102 at the intersection II.Regarding the Fink roof truss shown in FIG. 1 c, the expandableinterface 102 optionally is placed at intersections II and III of thefour web members 20.1 to 20.4 with the bottom chord 10, at intersectionsI, IV, and V of the four web members 20.1 to 20.4 with the top chords 12and 14 or, alternatively, only at intersection I of the web members 20.2and 20.3 with the top chords 12 and 14. Again, depending on the size,the design of the Fink roof truss and the lengthening of the top chords,it is possible to substantially reduce the uplift of the bottom chord 10by using only one expandable interface 102 at the intersection I.Regarding the Howe roof truss shown in FIG. 1 d, the expandableinterface 102 optionally is placed at intersections I, III and IV of thefive web members 20.1 to 20.5 with the bottom chord 10, at intersectionsII, V, and VI of the five web members 20.1 to 20.5 with the top chords12 and 14 or, alternatively, only at intersection II of the web member20.3 with the top chords 12 and 14. Again, depending on the size, thedesign of the Howe roof truss and the lengthening of the top chords, itis possible to substantially reduce the uplift of the bottom chord 10 byusing only one expandable interface 102 at the intersection 11.Regarding the Fan roof truss shown in FIG. 1 e, the expandable interface102 optionally is placed at intersections II and III of the six webmembers 20.1 to 20.6 with the bottom chord 10, at intersections I, IV,V, VI and VII of the six web members 20.1 to 20.6 with the top chords 12and 14 or, alternatively, at intersections I, IV and V of the four webmembers 20.2 to 20.5 with the top chords 12 and 14.

Of course, there are numerous other possibilities for placing theexpandable interface 102. It is noted that the expandable interface(s)is more effective in reducing uplift of the bottom chord 10 when placedsuch that the web members placed in closer proximity to a normal N tothe bottom chord 10 through the apex 16 are expandably interfaced thanthe web members placed at a larger distance to the normal N, as shown inthe example of a Double Fink roof truss in FIG. 3.

Alternatively, an expandable interface 103 is interposed between a firstportion 20A and a second portion 20B of an expandable web member of thewooden roof truss according to an embodiment of the invention, as shownin the simplified block diagram in FIG. 4. Here, the expandableinterface 103 enables relative movement of the first portion 20A withrespect to the second portion 20B substantially along a longitudinalaxis of the web member. By using the expandable interface 102 in thewooden roof truss according to the embodiment of the invention,expansion of the distance A—from distance A(1) to distance A(2)—isenabled and, therefore, uplift of the bottom chord is substantiallyreduced, as shown in FIG. 4.

As is evident, there are numerous possibilities of placing theexpandable interface 103 in the wooden roof truss according to theinvention. Regarding the Queen Post roof truss shown in FIG. 1 b, theexpandable interface 103 optionally is placed in the web member 20.2 or,alternatively, in all three web members 20.1 to 20.3. Regarding the Finkroof truss shown in FIG. 1 c, the expandable interface 103 optionally isplaced in the web members 20.2 and 20.3 or, alternatively, in all fourweb members 20.1 to 20.4. Of course, there are numerous otherpossibilities of placing the expandable interface 103, as well ascombinations with the expandable interface 102 above.

Typically, in North America wooden roof trusses are manufactured using astandard size of 2″×6″ for the chords, i.e. the chords have asubstantially rectangular cross section—oriented perpendicular to alongitudinal axis of the chord—of approximate size of ½″×3 ½″, and astandard size of 2″×4″ for the web members, i.e. the web members have asubstantially rectangular cross section—oriented perpendicular to alongitudinal axis of the web member—of size 1½″×2½″. The chords and webmembers are typically joined at the various intersections using “nailplates”—metal plates having integral teeth—nails—punched from the platematerial—which are placed on the front side and the backside of therespective components of the wooden roof truss to be joined and thenails are then driven into the wood using hydraulic clamps. While theinvention will be described hereinbelow based on this typicalmanufacturing process, it will become evident to those skilled in theart that the invention is not limited thereto but is also applicableusing different sizes and different methods of joining the componentssuch as, for example, using an adhesive.

Referring to FIGS. 5 a and 5 b, simplified block diagrams of front viewfirst embodiment of the expandable interface 102 according to theinvention—in an exemplary implantation for intersection I in FIG. 1 c—isshown. Here, the top chords 12 and 14 are joined using nail plates104—on the front and the back side—and the web members 20.2 and 20.3 arejoined using nail plates 106—on the front and the back side. It ispossible to use, for example, standard nail plates stamped from 16-,18-, or 20-gauge steel plates having integral nails 5/16″ to ⅜″ longwith approximately 8 nails per square inch, depending on forces to betransferred. By separately joining the top chords 12 and 14 and the webmembers 20.2 and 20.3 relative movement between the top chords 12 and 14and the web members 20.2 and 20.3 is enabled, as shown in FIG. 5 b. Ofcourse, the first embodiment of the expandable interface 102 is alsoapplicable for other intersections and roof truss shapes.

Referring to FIGS. 6 a to 6 d, simplified block diagrams of frontviews—FIGS. 6 a and 6 c—and cross sectional views—FIGS. 6 b and 6 d—of asecond embodiment of the expandable interface 102 according to theinvention—in an exemplary implantation for intersection I in FIG. 1 c—isshown. The top chords 12 and 14 are joined using nail plates 108—on thefront and the back side—and the web members 20.2 and 20.3 are joinedusing nail plates 106—on the front and the back side. Here, theexpandable interface 102 comprises a guiding mechanism for enablingguided movement of the web members 20.2 and 20.3 in a plane described bythe top chords 12 and 14 and the bottom chord 10. As shown in FIGS. 6 ato 6 g, the nail plates 108 comprise a mounting section 108 a and aguiding section 108 b with the guiding section 108 b protruding the topchords 12 and 14 and movable accommodating an end portion—indicated bydashed lines—of the web members 20.2 and 20.3 therebetween as shown inFIGS. 6 a to 6 d. The guiding sections 108 b are dimensioned such thatthe end portions of the web members 20.2 and 20.3 remain accommodatedtherebetween during a predetermined maximum relative movement D betweenthe top chords 12 and 14 and the web members 20.2 and 20.3. For example,it is possible to determine the maximum relative movement in dependenceupon the shape, the size, and the wooden material of the wooden rooftruss and a difference between the relative humidity experienced by thetop chords and the bottom chord. It has been found that in manysituations guided relative movement of approximately 1″ is sufficient.In order to movable accommodate the end portion of the web members 20.2and 20.3 between the guiding sections 108 b, the end portions comprise athickness smaller than a thickness of the chords 12 and 14. In case theweb members are made of material having a same thickness as the chords,the thickness of the end portion of the web members is reduced, as shownin FIGS. 6 b and 6 d, using for example a milling process.Alternatively, the guiding section 108 b of the nail plates 108 israised by a predetermined distance 110, as shown in a cross sectionalview of FIG. 6 f or, further alternatively, a raising layer 112 isinterposed between the mounting section 108 a of the nail plate 108 andthe respective web member, as shown in a cross sectional view of FIG. 6g. For example, the raising layer 112 is made of a material that ispenetrable by the nails—for example, a plastic material—or the layercomprises apertures for accommodating the nails therein. It is possibleto form the nail plate 108, for example, from standard 16-, 18-, or20-gauge steel plates using standard steel plate forming processes. Ofcourse, the second embodiment of the expandable interface 102 is alsoapplicable for other intersections and roof truss shapes such as, forexample, the intersection I in FIG. 1 a, shown in a front view in FIGS.6 h and 6 i, and the intersection I in FIG. 1 b, shown in FIGS. 6 j and6 k. Here, the mounting sections 108 a of the nail plates 108 aremounted to the web member 20—FIGS. 6 h and 6 i—or the web members 20.1to 20.3 for connecting the same—FIGS. 6 j and 6 k—while the guidingsections 108 b accommodate a potion of the respective chord 10therebetween.

Referring to FIGS. 7 a to 7 c, simplified block diagrams of frontviews—FIGS. 7 a and 7 c—and a cross sectional view—FIG. 7 b—of thirdembodiment of the expandable interface 102 according to the invention—inan exemplary implantation for intersection I in FIG. 1 b—is shown. Here,a bracket 120 comprising a mounting section 120 a and guiding sections120 b is mounted to the bottom chord 10 using, for example, nails orscrews. An end portion—indicated by dashed lines—of each of the webmembers 20.1 to 20.3 is movable accommodated in a respective guidingsection 120 b. The bracket 120 enables substantially independentrelative movement between each of the web members 20.1 to 20.3 and thebottom chord 10. As disclosed above with respect to the secondembodiment the guiding sections 120 b are designed to enable guidedmovement to a predetermined maximum distance. In order to facilitateimplementation with state of the art manufacturing processes, thebracket 120 is, for example, provided in the form of two nail plates122—one nail plate 122 mounted to the front side and one mounted to theback side of the bottom chord 10, as shown in a cross sectional view inFIG. 7 d. The nail plate 122 comprises a mounting section 122 a andraised guiding sections 122 b. The guiding sections 122 b each compriseguiding members 124 oriented substantially perpendicular to a plane ofthe mounting section 122 a in order to properly guide the respective webmember substantially parallel to its longitudinal axis. Again, it ispossible to form the nail plate 122, for example, from a standard 16-,18-, or 20-gauge steel plate using standard steel plate formingprocesses. Of course, the third embodiment of the expandable interface102 is also applicable for other intersections and roof truss shapes.

Referring to FIGS. 8 a to 8 c, simplified block diagrams of frontviews—FIGS. 8 a and 8 b—and a cross sectional view—FIG. 8 c—of theexpandable interface 103 according to an embodiment of the invention—inan exemplary implantation illustrated in FIG. 4—is shown. Here, theexpandable interface 103 is interposed between a first portion 20A and asecond portion 20B of the web member 20. The expandable interface 103enables relative movement of the first portion 20A with respect to thesecond portion 20B substantially along a longitudinal axis 132 of theweb member. Guiding is provided using two angled nail-plates 130—onenail plate 130 mounted to the front side and one mounted to the backside of the first portion 20A of the web member 20. The nail plate 130comprises a mounting section 130 a and a raised guiding section 130 b.The guiding section 130 b comprises a first plane portion 130 b. 1oriented substantially parallel to the mounting section and a secondplane portion 130 b.2 oriented substantially perpendicular to the firstplane portion 130 b. 1. The plane portions 130 b.I and 130 b.2 of thetwo nail plates 130 movable accommodate an end portion of the secondportion 20B of the web member 20 therebetween. It is possible to formthe nail plate 130, for example, from a standard 16-, 1 8-, or 20-gaugesteel plate using standard steel plate forming processes. Of course, theexpandable interface 103 is applicable for various web members of otherroof truss shapes. Alternatively, guiding is provided using a singlepiece—instead of the two angled plates 130—screwed or nailed to the endportion of first portion 20A of the web member 20.

It is noted, that those of skill in the art will readily arrive atnumerous other techniques to enable guided relative movement such as,for example, by providing a pin oriented parallel to the relativemovement which is accommodated in a respective bore.

The expandable interfaces disclosed above substantially reduce uplift ofthe bottom chord by enabling expansion of the distance A, while stillenabling transmission of compression forces in situations where therespective web member(s) are in contact with the corresponding chord(s),for example, when the top and the bottom chords are exposed to a samelevel of relative humidity or when a heavy snow load is causing the topchords to bend inward.

In some situations it is beneficial to limit the relative movementprovided by the expandable interfaces and to enable transmission oftension forces when the relative movement has reached a predeterminedlimit to counteract upward lifting forces acting on the top chords dueto, for example, strong wind forces acting on the roof in coldconditions.

Referring to FIGS. 9 a to 9 f, simplified block diagrams of frontviews—FIGS. 9 a and 9 f—and cross sectional views—FIGS. 9 b to 9 e—of afourth embodiment of the expandable interface 102 according to theinvention—in an exemplary implantation for intersection II in FIG. 1c—is shown. The web members 20.1 and 20.2 are joined using a firststructural member 140. The first structural member 140 comprises amounting section 140 a, for example, in the form of a nail plate, and aninteracting section 140 b protruding the web members 20.1 and 20.2 on aleft hand side and a right hand side. The interacting section 140 bcomprises an interacting element 140 b.1 oriented substantiallyperpendicular to the mounting section 140 a. Second structural members142, each comprising a mounting section 142 a and an U-shapedinteracting section 142 b, are mounted to the bottom chord 10 such thata substantially horizontal oriented portion of the U-shaped interactingsection 142 b of each of the second structural members 142 is able tointeract with a respective interacting element 140 b.1 in order to abutthe respective interacting element 140 b.1 when an upper limit of apredetermined range 144 has been reached, as shown in FIGS. 9 a to 9 c.When the upper limit has been reached, as shown in FIG. 9 c, theexpandable interface is capable of transmitting a tension force betweenthe web members 20.1 and 20.2 and the bottom chord via the abuttedinteracting element 140 b.1 and the substantially horizontal orientedportion of the U-shaped interacting section 142 b.

The first structural member 140 and the second structural members 142are dimensioned such that relative movement between the web members 20.1and 20.2 and the bottom chord is enabled within the predetermined rangeand that the interacting element 140 b.1 is abutted when the upper limitof the predetermined range has been reached. For example, it is possibleto determine the range of the relative movement in dependence upon theshape, the size, and the wooden material of the wooden roof truss and amaximum difference between the relative humidity experienced by the topchords and the bottom chord. It has been found that in many situationsrelative movement of approximately 1″ is sufficient. It is possible toform the first structural member 140 and the second structural members142, for example, from standard 16-, 18-, or 20-gauge steel plates usingstandard steel plate forming processes. For example, in order tofacilitate the roof truss manufacturing process the first structuralmember 140 and the second structural members 142 are provided in acombined fashion having the correct predetermined range 144 using, forexample an adhesive foil for holding the same in place priorinstallation. For installation the combined structure is placed onto thebottom chord 10 and the web members 20.1 and 20.2—with the web members20.1 and 20.2 being in contact with the bottom chord 10—such that thefirst structural member 140 is centered with the web members 20.1 and20.2 and a substantially horizontal portion of the U-shaped interactingsection 142 b of the second structural elements 142 is in contact with atop surface of the bottom chord and is then mounted using a hydraulicclamp. After mounting the adhesive foil is removed.

Provision of the first structural member 140 and the second structuralmembers 142 on the front side as well as on the backside of the woodenroof truss, as shown in FIG. 9 d, also provides guidance during therelative movement. Alternatively, the first structural member 140 andthe second structural members 142 are only mounted to the front sidewhile a guiding plate 146 comprising a mounting section 146 a and aguiding section 146 b is mounted to the backside of the web members 20.1and 20.2, as shown in FIG. 9 e. Further alternatively, the firststructural member 140 comprises a single interacting section 140 b forinteracting with a single second structural member 142. Provision ofsame first structural members 140 comprising a single interactingsection 140 b and corresponding second structural members 142 on thefront side as well as on the backside, as shown in FIG. 9 f, alsoprovides guidance during the relative movement.

Optionally, second structural members 148, each comprising a mountingsection 148 a and an L-shaped interacting section 148 b, are mounted tothe opposite side of the bottom chord 10 such that a substantiallyhorizontal oriented portion of the L-shaped interacting section 148 b ofeach of the second structural members 148 is able to interact with arespective interacting element 140 b.1 in order to abut the same when anupper limit of the predetermined range 144 has been reached, as shown inFIG. 9 g. The second structural member 148 is applicable, for example,in the variations shown in FIGS. 9 e and 9 f.

Of course, the fourth embodiment of the expandable interface 102—and itsvariations—is also applicable for other intersections and roof trussshapes.

Referring to FIGS. 10 a to 10 d, a variation of the third embodiment ofthe expandable interface 102 comprising a limiting mechanism is shown.Here, the end portion of the web member 20.2 comprises an elongatedaperture or groove 150 which is oriented substantially parallel to alongitudinal axis 151 of the web member 20.2, as shown in a crosssectional view illustrated in FIG. 10 b and a front view of the webmember 20.2. The aperture or groove 150 is terminated at termination 152which is placed at a predetermined location along the longitudinal axis151. A limiting element 154 is connected to the guiding section 122 b ofthe bracket 122—which is provided in the form of two nail plates asdescribed above—for abutting the termination 152 when the relativemovement has reached a predetermined limit, as shown in FIG. 10 d. Thelimiting element 154 is, for example, made as a punched and bent portionof the guiding section 122 b, as shown in FIGS. 10 a, 10 b, and 10 d.Alternatively, the web member 20.2 comprises an elongated aperture 150and the limiting element 154 is a pin accommodated in respectiveapertures of the guiding sections 120 b, 122 b and fastened thereto, asshown in FIG. 10 e. Further alternatively, the guiding section 122 bcomprises an elongated aperture 158 which is oriented substantiallyparallel to the longitudinal axis 151 of the web member 20.2 andcomprises a termination 159 placed at a predetermined location along thelongitudinal axis 151, as shown in FIGS. 10 f and 10 g. A limitingelement 162 such as, for example, a pin is connected—for example, byusing a tight fit with a respective bore—to the end portion of the webmember 20.2 for being abutted by the termination when the relativemovement has reached a predetermined limit. For example, it is possibleto determine the locations of the terminations and the limiting elementsin dependence upon the predetermined limit of the relative. In numerousapplications it is sufficient to provide a single limiting mechanism forthe bracket 120, 122, as shown, but as is evident, it is possible toemploy the limiting mechanism for limiting the relative movement of morethan one web member.

The same variations as shown in FIGS. 10 a to 10 g are also applicablefor limiting the relative movement of the first portion 20A with respectto the second portion 20B of the web member 20 in the expandableinterface 103 shown in FIGS. 8 a to 8 c. Furthermore, these variationsare also applicable as limiting mechanism in the second embodiment ofthe interface 102 shown in FIGS. 6 a to 6 k.

In various situations it is sufficient to provide a single expandableinterface of a plurality of expandable interfaces with a limitingmechanism. For example, the roof truss shown in FIG. 1 d is providedwith expandable interfaces at the intersections I, III, and IV of whichthe expandable interface at intersection I comprises a limitingmechanism.

Numerous other embodiments of the invention will be apparent to personsskilled in the art without departing from the spirit and scope of theinvention as defined in the appended claims.

1. A wooden roof truss comprising: a bottom chord, a first top chord, and a second top chord, the bottom chord, the first top chord, and the second top chord joined together forming a triangle with an apex spaced at a distance A from the bottom chord; a web member disposed between the top chords and the bottom chord such that, in a first state of operation, the web member transmits one of a tension force and a compression force between the top chords and the bottom chord; and, an expandable interface for expandably interfacing the web member with one of: the first top chord; the second top chord; the first and the second top chord; and, the bottom chord, wherein the expandable interface supports, in a second state of operation, variation of the distance A between the apex and the bottom chord.
 2. A wooden roof truss as defined in claim 1 wherein the expandable interface comprises a guiding mechanism for enabling guided movement of the expandably interfaced web member in a plane described by the first top chord, the second top chord, and the bottom chord.
 3. A wooden roof truss as defined in claim 2 wherein the guiding mechanism comprises a first plate and a second plate, the first plate and the second plate each comprising a mounting section and a guiding section, wherein the mounting sections of the first plate and the second plate are mounted to a front side and a back side, respectively, of the expandably interfaced web member, and wherein the guiding sections of the first plate and the second plate movably accommodate a portion of the chord to which the web member is expandably interfaced therebetween.
 4. A wooden roof truss as defined in claim 3 wherein the mounting section of the first plate and the second plate is used to connect one of: at least two expandable interfaced web members; and, the first and the second top chord.
 5. A wooden roof truss as defined in claim 3 wherein the mounting section of the first plate and the second plate forms a nail plate.
 6. A wooden roof truss as defined in claim 1 wherein the expandable interface comprises a limiting mechanism for limiting movement of the expandably interfaced web member relative to the chord to which the web member is expandably interfaced.
 7. A wooden roof truss as defined in claim 6 wherein the limiting mechanism comprises a first structural member mounted to the expandably interfaced web member and a second structural member mounted to the chord to which the web member is expandably interfaced, the first structural member movable relative to the second structural member within a predetermined range and for being abutted by the second structural member when an upper limit of the predetermined range has been reached.
 8. A wooden roof truss as defined in claim 7 wherein the first structural member comprises a first plate having a first mounting section mounted to one of a front side and a back side of the expandably interfaced web member and having a first interacting section oriented substantially perpendicular to the first mounting section, and wherein the second structural member comprises a second plate having a second mounting section mounted to one of a front side and a back side of the chord to which the web member is expandably interfaced and having a second interacting section oriented substantially perpendicular to the second mounting section.
 9. A wooden roof truss as defined in claim 8 wherein the second structural member comprises one of: an L shape with the second mounting section mounted to a side opposite to the side the first mounting section is mounted; and, a U shaped interacting section with the second mounting section mounted to a same side the first mounting section is mounted.
 10. A wooden roof truss as defined in claim 8 wherein the first mounting section is for connecting two expandably interfaced web members.
 11. A wooden roof truss as defined in claim 8 wherein the first mounting section forms a nail plate.
 12. A wooden roof truss as defined in claim 8 comprising a guiding plate mounted to a side opposite the first plate, the guiding plate comprising a mounting section and a guiding section with the guiding section protruding from the expandably interfaced web member and overlapping a portion of one side of the chord the web member is expandably interfaced with.
 13. A wooden roof truss as defined in claim 8 wherein the first structural member comprises a front first plate mounted to the front side of the expandably interfaced web member and a back first plate mounted to the back side of the expandably interfaced web member, and wherein the second structural member comprises a front second plate mounted to a front side of the chord to which the web member is expandably interfaced and a back second plate mounted to a back side of the chord to which the web member is expandably interfaced.
 14. A wooden roof truss as defined in claim 7 wherein the first structural member comprises a first plate having a first mounting section mounted to one of a front side and a back side of the expandably interfaced web member, a left hand side first interacting section and a right hand side first interacting section, the left hand side and the right hand side first interacting sections being oriented substantially perpendicular to the first mounting section, and wherein the second structural member comprises a left hand side second plate and a right hand side second plate, the left hand side and the right hand side second plates each having a second mounting section mounted to one of a front side and a back side of the chord to which the web member is expandably interfaced and a second interacting section oriented substantially perpendicular to the second mounting section.
 15. A wooden roof truss as defined in claim 1 wherein the expandable interface is placed such that the expandably interfaced web member is a web member placed closer to a normal to the bottom chord through the apex than web members that are other than expandably interfaced.
 16. A wooden roof truss comprising: a bottom chord, a first top chord, and a second top chord, the bottom chord, the first top chord, and the second top chord joined together forming a triangle with an apex spaced at a distance A from the bottom chord; a web member disposed between at least one of the top chords and the bottom chord such that, in a first state of operation, the web member transmits one of a tension force and a compression force between the at least one of the top chords and the bottom chord, wherein the web member comprises an expandable interface disposed between a first and a second portion of the web member for supporting relative movement of the first portion with respect to the second portion substantially along a longitudinal axis of the web member for supporting, in a second state of operation, variation of the distance A between the apex and the bottom chord.
 17. A wooden roof truss as defined in claim 16 wherein the expandable interface comprises a guiding mechanism mounted to an end portion of the first portion of the web member for supporting the relative movement in a guided fashion.
 18. A wooden roof truss as defined in claim 17 wherein the guiding mechanism comprises a first and a second angle plate, each of the first and the second angle plate comprising a first plane portion and a second plane portion oriented substantially perpendicular to the first plane portion, each of the first and the second angle plate comprising a mounting section mounted to an end portion of the first portion of the web member and an angled guiding section for movably accommodating an end portion of the second portion of the web member.
 19. A wooden roof truss as defined in claim 16 wherein the expandable interface comprises a limiting mechanism for limiting the relative movement.
 20. A wooden roof truss as defined in claim 19 wherein the limiting mechanism comprises one of a first limiter and a second limiter, the first limiter comprising: one of an elongated aperture and a groove disposed in an end portion of the second portion of the web member, the one of an elongated aperture and a groove being oriented substantially parallel to a longitudinal axis of the web member and comprising a termination placed at a predetermined location along the longitudinal axis; and, a limiting element connected to the guiding mechanism for abutting the termination when the relative movement has reached a predetermined limit, and the second limiter comprising; an elongated aperture disposed in the guiding mechanism oriented substantially parallel to the longitudinal axis of the web member and comprising a termination placed at a predetermined location along the longitudinal axis of the web member; and, a limiting element connected to the end portion of the second portion of the web member for being abutted by the termination when the relative movement has reached a predetermined limit.
 21. A wooden roof truss as defined in claim 20 wherein the expandable interface comprises a bracket mounted to at least a chord that the web member is expandably interfaced with for enabling relative movement between each of the web member and the at least a chord.
 22. A wooden roof truss as defined in claim 21 wherein the bracket comprises a mounting section mounted to the at least a chord and a guiding section for the web member for movably accommodating an end portion of the web member therein.
 23. A wooden roof truss as defined in claim 22 wherein the bracket comprises a front plate mounted to a front side and a back plate mounted to a back side of the at least a chord, the front plate and the back plate each comprising a substantially plane mounting section and a guiding section, the guiding section of at least one of the front plate and the back plate comprising at least a guiding member oriented substantially perpendicular to a plane of the mounting section.
 24. A wooden roof truss as defined in claim 22 wherein the expandable interface comprises a limiting mechanism for limiting movement of the expandably interfaced web member relative to the chord to which the web member is expandably interfaced.
 25. A wooden roof truss as defined in claim 24 wherein the limiting mechanism comprises one of a first limiter and a second limiter, the first limiter comprising: one of an elongated aperture and a groove disposed in an end portion of a respective web member, the one of an elongated aperture and a groove being oriented substantially parallel to a longitudinal axis of the web member and comprising a termination placed at a predetermined location along the longitudinal axis; and, a limiting element connected to the bracket for abutting the termination when the relative movement has reached a predetermined limit, and the second limiter comprising; an elongated aperture disposed in the guiding section of the bracket oriented substantially parallel to a direction of the relative movement of the web member and comprising a termination placed at a predetermined location along the direction of the relative movement; and, a limiting element connected to an end portion of the web member for being abutted by the termination when the relative movement has reached a predetermined limit. 